Activation and transdominant suppression of MHC class II and HLA-DMB promoters by a series of C-terminal class II transactivator deletion mutants.

The class II transactivator (CIITA) is a highly specific transcription factor that activates only genes known to be involved in the class II MHC processing pathway, including class II MHC, invariant chain, and HLA-DMA/B genes. In this work, we show the requirement of a new region in CIITA that is critical for its function. Deletion mutants lacking varying length of the C terminus show that the C-terminal 41 amino acids of CIITA are indispensable for the activation of both the conventional DRA and nonconventional DMB promoters. This region contains a highly charged stretch of amino acids that is homologous to a yeast transcription factor, repression activator protein-1 (RAP1)-interacting factor 1. Mutants lacking the C terminus were tested in a transdominant-negative assay to examine their capacity to block the wild-type CIITA function. Two of these deletion mutants suppressed the activity of endogenously expressed wild-type CIITA to activate both DRA and DMB promoters. It may be possible to utilize these mutants to modulate MHC class II and DM gene expression.